Jong-Kook Lee

Sinoatrial Node Dysfunction and Early Unexpected Death of Mice With a Defect of klotho Gene Expression

Background—Homozygous mutant mice with a defect of klotho gene expression (kl/kl) show multiple age-related disorders and premature death from unknown causes. Methods and Results—The kl/kl mice subjected to 20-hour restraint stress showed a high rate (20/30) of sudden death, which was associated with sinoatrial node dysfunction (conduction block or arrest). Heart rate and plasma norepinephrine of kl/kl mice, unlike those of wild-type (WT) mice, failed to increase during the stress. Intrinsic heart rate after pharmacological blockade of autonomic nerves in kl/kl mice was significantly lower than that in WT mice (380±33 versus 470±44 bpm; n=7). The sinus node recovery time after an overdrive pacing (600 bpm, 30 seconds) in kl/kl mice was significantly longer than in WT mice (392±37 versus 233±24 ms; n=6). In isolated sinoatrial node preparations, the positive chronotropic effect of isoproterenol was significantly less, whereas the negative chronotropic effect of acetylcholine was significantly greater in kl/kl than in WT mice. There was no degenerative structural change in the sinoatrial node of kl/kl mice. The precise localization of klotho was analyzed in newly prepared klotho-null mice with a reporter gene system (kl−geo). Homozygous kl− geo mice showed characteristic age-associated phenotypes that were almost identical to those of kl/kl mice. In the kl− geo mice, klotho expression was recognized exclusively in the sinoatrial node region in the heart in addition to parathyroid, kidney, and choroid plexus. Conclusions—In the heart, klotho is expressed solely at the sinoatrial node. klotho gene expression is essential for the sinoatrial node to function as a dependable pacemaker under conditions of stress.

Sema3a maintains normal heart rhythm through sympathetic innervation patterning

Sympathetic innervation is critical for effective cardiac function. However, the developmental and regulatory mechanisms determining the density and patterning of cardiac sympathetic innervation remain unclear, as does the role of this innervation in arrhythmogenesis. Here we show that a neural chemorepellent, Sema3a, establishes cardiac sympathetic innervation patterning. Sema3a is abundantly expressed in the trabecular layer in early-stage embryos but is restricted to Purkinje fibers after birth, forming an epicardial-to-endocardial transmural sympathetic innervation patterning. Sema3a−/− mice lacked a cardiac sympathetic innervation gradient and exhibited stellate ganglia malformation, which led to marked sinus bradycardia due to sympathetic dysfunction. Cardiac-specific overexpression of Sema3a in transgenic mice (SemaTG) was associated with reduced sympathetic innervation and attenuation of the epicardial-to-endocardial innervation gradient. SemaTG mice demonstrated sudden death and susceptibility to ventricular tachycardia, due to catecholamine supersensitivity and prolongation of the action potential duration. We conclude that appropriate cardiac Sema3a expression is needed for sympathetic innervation patterning and is critical for heart rate control.

Complement C1q Activates Canonical Wnt Signaling and Promotes Aging-Related Phenotypes

Wnt signaling plays critical roles in development of various organs and pathogenesis of many diseases, and augmented Wnt signaling has recently been implicated in mammalian aging and aging-related phenotypes. We here report that complement C1q activates canonical Wnt signaling and promotes aging-associated decline in tissue regeneration. Serum C1q concentration is increased with aging, and Wnt signaling activity is augmented during aging in the serum and in multiple tissues of wild-type mice, but not in those of C1qa-deficient mice. C1q activates canonical Wnt signaling by binding to Frizzled receptors and subsequently inducing C1s-dependent cleavage of the ectodomain of Wnt coreceptor low-density lipoprotein receptor-related protein 6. Skeletal muscle regeneration in young mice is inhibited by exogenous C1q treatment, whereas aging-associated impairment of muscle regeneration is restored by C1s inhibition or C1qa gene disruption. Our findings therefore suggest the unexpected role of complement C1q in Wnt signal transduction and modulation of mammalian aging.

Chamber-specific differentiation of Nkx2.5-positive cardiac precursor cells from murine embryonic stem cells

Embryonic stem (ES) cells are a useful system to study cardiac differentiation in vitro. It has been difficult, however, to track the fates of chamber‐specific cardiac lineages, since differentiation is induced within the embryoid body. We have established an in vitro culture system to track Nkx2.5(+) cell lineages during mouse ES cell differentiation by using green fluorescent protein (GFP) as a reporter. Nkx2.5/GFP(+) cardiomyocytes purified from embryoid bodies express sarcomeric tropomyosin and myosin heavy chain and heterogeneously express cardiac troponin I (cTnI), myosin light chain 2v (MLC2v) and atrial natriuretic peptide (ANP). After 4‐week culture, GFP(+) cells exhibited electrophysiological characteristics specific to sinoatrial (SA) node, atrial, or ventricular type. Furthermore, we found that administration of 10−7 M retinoic acid (RA) to embryoid bodies increased the percentage of MLC2v(−)ANP(+) cells; this also increased the expression of atrial‐specific genes in the Nkx2.5/GFP(+) fraction, in a time‐ and dose‐dependent fashion. These results suggest that Nkx2.5(+) lineage cells possess the potential to differentiate into various cardiomyocyte cell types and that RA can modify the differentiation potential of Nkx2.5(+) cardiomyocytes at an early stage.

Identification of Human Kinases Involved in Hepatitis C Virus Replication by Small Interference RNA Library Screening

The propagation of the hepatitis C virus (HCV) is a complex process that requires both host and viral proteins. To facilitate identification of host cell factors that are required for HCV replication, we screened a panel of small interference RNAs that preferentially target human protein kinases using an HCV replicon expressing the firefly luciferase gene as a genetic reporter. Small interference RNAs specific for three human kinases, Csk, Jak1, and Vrk1, were identified that reproducibly reduce viral RNA and viral protein levels in HCV replicon-bearing cells. Treatment of replicon cells with a small molecule inhibitor of Csk also resulted in a significant reduction in HCV RNA and proteins, further supporting a role for Csk in HCV replication. The effects of siRNAs targeting eight kinases known to be negatively regulated by Csk were then examined; knock down of one of these kinases, Fyn, resulted in up-regulation of the HCV replicon, suggesting that Csk mediates its effect on HCV replication through Fyn. This conclusion was further corroborated by demonstration that replicon cells treated with Csk inhibitor contained lower levels of the phosphorylated form of Fyn than control cells.

Developmental changes of Ca2+ handling in mouse ventricular cells from early embryo to adulthood

Transplant of immature cardiomyocytes is recently attracting a great deal of interest as a new experimental strategy for the treatment of failing hearts. Full understanding of normal cardiomyogenesis is essential to make this regenerative therapy feasible. We analyzed the molecular and functional changes of Ca(2+) handling proteins during development of the mouse heart from early embryo at 9.5 days postcoitum (dpc) through adulthood. From the early to the late (18 dpc) embryonic stage, mRNAs estimated by the real time PCR for ryanodine receptor (type 2, RyR2), sarcoplasmic reticulum (SR) Ca(2+) pump (type 2, SERCA2) and phospholamban (PLB) increased by 3-15 fold in the values normalized to GAPDH mRNA, although Na(+)/Ca(2+) exchanger (type 1, NCX1) mRNA was unchanged. After birth, there was a further increase in the mRNAs for RyR2, SERCA2 and PLB by 18-33 fold, but a 50% decrease in NCX1 mRNA. The protein levels of RyR2, SERCA2, PLB and NCX1, which were normalized to total protein, showed qualitatively parallel developmental changes. L-type Ca(2+) channel currents (I(Ca-L)) were increased during the development (1.3-fold at 18 dpc, 2.2-fold at adult stage, vs. 9.5 dpc). At 9.5 dpc, the Ca(2+) transient was, unlike adulthood, unaffected by the SR blockers, ryanodine (5 microM) and thapsigargin (2 microM), and also by a blocker of the Ca(2+) entry via Na(+)/Ca(2+) exchanger, KB-R 7943 (1 microM). The Ca(2+) transient was abolished after application of nisoldipine (5 microM). These results indicate that activator Ca(2+) for contraction in the early embryonic stage depends almost entirely on I(Ca-L).

Ionic Mechanisms of Acquired QT Prolongation and Torsades de Pointes in Rabbits With Chronic Complete Atrioventricular Block

Background—The ionic basis of acquired QT prolongation and torsade de pointes (TdP) unrelated to drugs is not fully understood. Methods and Results—We created a rabbit model with chronic complete atrioventricular block (AVB) (n=34), which showed prominent QT prolongation (by 120%), high incidence of spontaneous TdP (71%), and cardiac hypertrophy. Patch-clamp experiments were performed in left ventricular myocytes from 9 rabbits (8 with TdP, 1 without TdP) at ≈21 days of AVB and from 8 sham-operated controls with sinus rhythm. Action potential duration was prolonged in AVB myocytes compared with control (+61% at 0.5 Hz, +21% at 3 Hz). Both rapidly and slowly activating components of the delayed rectifier K+ current (IKr and IKs) in AVB myocytes were significantly smaller than in control by 50% and 55%, respectively. There was no significant difference in Ca2+-independent transient outward current (Ito1). L-type Ca2+ current (ICa,L) in control and AVB myocytes was similar in peak amplitude, but the half voltage for activation was shifted to the negative direction (5.9 mV) in AVB myocytes. Voltage dependence of ICa,L inactivation was not different in control and AVB myocytes. The inward rectifier K+ current (IK1) significantly increased in AVB myocytes compared with control. Conclusions—In the rabbit, chronic AVB leads to prominent QT prolongation and high incidence of spontaneous TdP. Downregulation of both IKr and IKs in association with altered ICa,L activation kinetics may underlie the arrhythmogenic ventricular remodeling.

Midkine Plays a Protective Role Against Cardiac Ischemia/Reperfusion Injury Through a Reduction of Apoptotic Reaction

Background— Midkine (MK) is a heparin-binding growth factor involved in diverse biological phenomena, eg, neural survival, carcinogenesis, and tissue repair. MK could have a protective action against ischemia/reperfusion (I/R) injury in the heart, because MK was shown to have cytoprotective activity in cultured neurons and tumor cells. We investigated this hypothesis in mice with and without genetic MK deletion. Methods and Results— Myocardial injury after I/R was produced by transient occlusion of coronary arteries. In wild-type (Mdk+/+) mice, MK expression was increased after I/R in the periinfarct area. Infarct size/area at risk 24 hours after I/R in MK-deficient (Mdk−/−) mice was larger than in Mdk+/+ mice (55.4±9.1% versus 32.1±5.3%, P<0.05). Terminal dUTP nick end-labeling–positive myocyte population in the periinfarct area in Mdk−/− mice was higher than in Mdk+/+ mice (6.8±0.9% versus 3.2±0.6%, P<0.05). Left ventricular fractional shortening 24 hours after I/R in Mdk−/− mice was significantly less than that in Mdk+/+ mice (34.3±4.4% versus 50.8±2.1%, P<0.05). Supplemental application of MK protein to left ventricle of Mdk−/− mice at the time of I/R resulted in reduction of the infarct size. Application of exogenous MK to cultured cardiomyocytes resulted in increased Bcl-2 expression and decreased apoptosis after hypoxia/reoxygenation. Conclusions— These results suggest that MK plays a protective role against I/R injury, most likely through a prevention of apoptotic reaction. MK is a potentially important new molecular target for treatment of ischemic heart disease.

Cell-to-cell interaction prevents cell death in cultured neonatal rat ventricular myocytes

OBJECTIVES Loss of cardiac cells and the anatomical or functional remodeling of intercellular coupling occur under several pathological conditions. We have assessed the significance of intercellular coupling for cell death. METHODS AND RESULTS Ventricular cells obtained from 1 day old Wistar rats were cultured. Apoptosis was detected by nick-end labeling. Cells were plated at low and high cell density (3x10(4)/ml and 12x10(4)/ml, respectively). Cultured myocytes died spontaneously by apoptosis in a time dependent manner. The increase of the apoptotic cell population in a culture with high cell density on day 4 (1+/-1.2%, n=4) was significantly lower than that in a culture with low cell density (20+/-5.5%, n=4). The progression of apoptosis in the culture of low cell density was prevented in part after application of the medium extract from the culture of high cell density; the apoptotic cell population on day 6 decreased from 57+/-8.0% (n=4) to 36+/-3.8% (n=4). Treatment of the cultured myocytes at high cell density with antisense oligonucleotide for connexin43 (Cx43) for 24 h on day 2 resulted in a significant decrease in Cx43 expression as judged by Western blot, dye transfer and immunocytochemistry using mouse monoclonal antibody for Cx43. In association with the down-regulation of Cx43, the progress of apoptosis was accelerated; the apoptotic cell population on day 5 in the antisense-treated cultures (27+/-5.7%, n=4) was significantly higher than the sense-treated cultures (5+/-1.1%, n=4). The effect of Cx43 antisense treatment to promote apoptosis was not reversed by application of high cell-density culture medium. CONCLUSIONS These findings suggest that cell-cell communication through gap junction formation and some humoral factors play important roles in the survival of cultured myocytes.

Contribution of hydrophobic and electrostatic interactions to the membrane integration of the Shaker K+ channel voltage sensor domain

Membrane-embedded voltage-sensor domains in voltage-dependent potassium channels (Kv channels) contain an impressive number of charged residues. How can such highly charged protein domains be efficiently inserted into biological membranes? In the plant Kv channel KAT1, the S2, S3, and S4 transmembrane helices insert cooperatively, because the S3, S4, and S3–S4 segments do not have any membrane insertion ability by themselves. Here we show that, in the Drosophila Shaker Kv channel, which has a more hydrophobic S3 helix than KAT1, S3 can both insert into the membrane by itself and mediate the insertion of the S3–S4 segment in the absence of S2. An engineered KAT1 S3–S4 segment in which the hydrophobicity of S3 was increased or where S3 was replaced by Shaker S3 behaves as Shaker S3–S4. Electrostatic interactions among charged residues in S2, S3, and S4, including the salt bridges between E283 or E293 in S2 and R368 in S4, are required for fully efficient membrane insertion of the Shaker voltage-sensor domain. These results suggest that cooperative insertion of the voltage-sensor transmembrane helices is a property common to Kv channels and that the degree of cooperativity depends on a balance between electrostatic and hydrophobic forces.